In continuous ink jet printing systems, the eyelid is a moveable seal which diverts ink on startup into the catcher, thereby recycling the ink while containing it within the printhead. The seal is formed against the lip of the metal catch plate, which is typically about 0.025 inches thick. The eyelid opens about 0.04 inches while the printer is printing, allowing the ink drops to pass onto the print media. The area behind the eyelid, containing the droplet generator and charging leads, is frequently accessed for cleaning over the life of a printhead.
The printhead is typically located over the top of a roller carrying the print media, with the ink drops moving downward in a roughly vertical direction. To assure good print quality, the printhead is 0.075 to 0.100 inches above the substrate. The eyelid must seal perpendicular to the drop path while not contacting the moving substrate on the roller. The motion paths of prior art have been limited to a linear sliding, or a single pivot. The linear paths of some prior art are guided by slots which are prone to collecting ink residue, leading to sticking or jamming of the eyelid.
Prior eyelids have used simple spring loaded solenoid actuators. The spring maintains the seal force until the solenoid opens it to a print position. However, the solenoid may create an excessive shock when the eyelid opens and impacts its print position stop. This shock jars the ink jet printhead, causing the printhead to malfunction. To prevent this, cushioning springs or rubber dampers are used to buffer the impact of the eyelid and solenoid plunger at the end of its travel.
Both the linear slide and the simple pivot eyelids are suitable for a seal moving a small distance from the sealed position to the print position. These configurations, however, do not move far enough to allow the operator to look at the jets and charging electrodes in a horizontal direction while they are cleaning the printhead. It is therefore necessary for the eyelid to be removed by a trained operator for printhead cleaning. This may entail removing screws and covers, necessitating the use of a tool for fasteners and the possibility that the parts may be lost or damaged on reassembly. Proper alignment of the sealing edge with the catchplate may be difficult to obtain after repeated disassembly, leading to leaks or poor startups.
An additional safety issue exists, since the printhead contains high voltage (˜150 Volts). While operating, it is necessary to maintain proper ingress protection to avoid accidental contact with the charging electrodes. In the existing art the removal of covers by trained service personnel is the only means provided to prevent accidental access to the electrodes while the high voltage is on.
As mentioned earlier, it is possible for the eyelid to become stuck or jammed. The prior art, with a simple solenoid actuator, has no feedback of eyelid position or presence. If the eyelid jams while closing, no indication is made to the system of the malfunction. This could result in ink spraying out of the printhead during start up or shut down.
One existing patent, U.S. Pat. No. 6,247,781, described an improved eyelid mechanism to overcome some of the problems of other prior art systems. The '781 patent showed an eyelid that could be positioned, using a stepper motor, into the three positions of closed, print and service. The eyelid closed position seals the eyelid against the catcher to prevent ink from leaking from the printhead during start up and shutdown operations. The print position moves the eyelid seal away from the catcher to allow print drops to pass between the eyelid and the catcher on their way to the print media. A service position moves the eyelid much farther away from the catcher, providing a printhead operator better access for cleaning the printhead orifice plate and charge plate. It employs electro mechanical switches and a stepper motor drive to facilitate movement of the eyelid to those three positions. During motion of the eyelid, the eyelid is prevented from making contact with the paper. The stepper motor drives the eyelid into the three positions. Switch means detect that the eyelid is properly actuated to the desired locations.
While the eyelid mechanism of the '781 was effective in providing the three operating positions, it has been found that the switch means for detecting the eyelid position had certain drawbacks. First, the switch means has proven to be cumbersome to locate properly on the eyelid. Misplacement of the switches could result in either insufficient or excessive sealing forces being supplied to seal against the catcher. Second, the reliability of the switches exposed to the operating environment of the inkjet printhead can be inadequate. Third, the placement of the switches may interfere with the desired flow of air through the printhead. Fourth, a primary function of the switches was to detect possible eyelid actuation errors. The most common error has been a failure of the eyelid to open, due to the eyelid sticking to the catcher pan in the closed position.
It would be desirable to provide an improved eyelid actuation means to provide for the required three operating positions, that overcomes the problems associated with the prior art.